1. Field of the Invention
The present invention relates generally to the field of processing of borehole seismic data. More specifically, the present invention discloses a method for processing borehole seismic data into the form of surface seismic data so that conventional surface seismic data processing methods can then be applied to form a subsurface image of the earth.
2. Statement of the Problem
The widely known and widely used art of surface seismology involves placing seismic sources and seismic receivers at the surface of the earth and recording seismic waves that originate at the seismic source point. As illustrated in FIG. 1, a conventional method of collecting seismic data in surface seismic operations is to place seismic sources and seismic receivers at the surface of the earth. Each seismic source is initiated and the seismic wavefield from the individual seismic sources is recorded on multiple receivers. Data recorded by geophones (also known as seismometers) at the surface of the earth can be processed by widely known methods (see, Yilmaz, O., Seismic Data Processing, (Society of Exploration Geophysicists, 1987)) to obtain an image of the interior of the earth.
One of the most commonly used processing methods is called CDP (Common Depth Point) processing. In this method, sources and receivers from different common source gathers (see FIG. 1) are sorted into common midpoint gathers, otherwise known as CDP (Common Depth Point) gathers, as illustrated in FIG. 2.
Reflections in a CDP gather are hyperbolic in the time-offset plane, as shown in FIG. 3, where the word “offset” is used to describe the horizontal distance from the source to the receivers. The trace on the left side of the gather in FIG. 3 has an offset of zero, in other words the source and receiver were coincident in space at the time of recording. The time delay of reflections with increasing offset is due to the increased seismic wave travel path with increased source-receiver separation in the horizontal direction.
A mathematical operation known as Normal Moveout (NMO) can be applied to the reflections in a CDP gather to correct reflection travel times so that the reflection time after application of NMO is equivalent to the travel time at zero-offset, i.e., where the source and receiver were coincident at the surface of the earth at the time of recording. FIG. 4 shows a synthetic common depth point gather in FIG. 3 after correction for NMO.
Having both sources and receivers at the surface of the earth is required for two key aspects of this reflection seismology technique to work properly. The two aspects are: (1) to first order, the spatial point from which a seismic reflection originates can be assumed to be half way between the source and receiver; and (2) the shape of a reflection in the time-offset plane is hyperbolic and can be predicted by the NMO equation. The assumptions of these two key aspects are violated in proportion to the degree that reflecting interfaces in the subsurface dip (or tilt) from flat lying. But even with steep dips, the earth can be imaged with well-developed surface seismic techniques.
A sub-field of reflection seismology is borehole seismology in which seismic receivers are placed in one or more boreholes in the subsurface and source points are at the surface of the earth, as shown in FIG. 5. This type of data is generally known as Offset VSP (Vertical Seismic Profile) data, but is also alternatively known as 2D VSP or 3D VSP data. Alternatively the source can be in the borehole with receivers at the surface of the earth. The borehole seismic source can be of any type, including data derived from using a drilling bit as the seismic source. This technique is commonly known as Reverse VSP.
There are significant advantages to recording seismic data by VSP methods, not the least of which is increased seismic frequency content over that which can be recorded at the surface of the earth. Therefore, the potential exists to obtain greater geologic detail from the data. The significant disadvantage however is that the symmetry of having source and receivers at the same elevation is lost. Thus, the common midpoint reflection point assumption is lost and the NMO equation does not apply. Further, there is not currently an analogous equation for midpoint determination and moveout correction to apply to offset VSP data.
Solution to the Problem. The present invention provides a method that enables borehole seismic data (e.g., VSP data) to be transformed into a form such that the data is as though it had been recorded with seismometers at the surface of the earth. After this transformation, the data can be processed as though it had been originally recorded at the surface of the earth using well-developed methods of surface seismic data processing. The transformation is achieved by continuation of the wavefield in time to a number of pseudo-receivers located at the surface. Wavefield continuation can be achieved via multiple methods, two of which are described below as alternative methods that achieve the desired goal of this invention.
The prior art in the general field of wavefield propagation includes scientific literature containing fundamental ideas that naturally spring from the mathematics that describe elastic wave motion. For example, Huygens' principle provides that the position of a subsequent wavefront may be found by regarding each point of an earlier wavefront as a source of spherical secondary waves whose envelope constitutes the new wavefront. Elmore and Heald, Physics of Waves, page 323 (Dover Publications, 1969).
Specific techniques that make use of wavefield continuation in the field of seismic data analysis include upward continuation of surface seismic data from the surface of the earth to relatively near-by artificial planes for the purpose of relieving statics problems. U.S. Pat. No. 5,629,905 (Lau) discloses a method for downward continuation of surface seismic data to arbitrary subsurface planes for the purposes of improved imaging of complex surfaces. Specifically related to borehole seismic data, Ala'i, Riaz, Improving Predrilling Views By Pseudo Seismic Borehole Data, Ph.D. thesis, Delft University of Technology, The Netherlands. (1997), showed the concept of transforming surface seismic data into zero-offset VSP seismic data by downward continuation of the surface seismic wavefield.
None of these methods either implicitly or explicitly addresses the process of upward continuation of borehole seismic data for the purpose of transforming the data into a form suitable for use with surface seismic data processing techniques. Therefore, a need exists for a method of processing offset VSP data that is not limited by the asymmetry imposed by the original source-receiver recording geometry that includes the sources and receivers at very different elevations at the time of recording.